histamine /his″təmēn, -min/ [Gk, histos, tissue; L, amine, ammonia] , a compound, found in all cells, that is produced by the breakdown of histidine. It is released in allergic inflammatory reactions. Cellular receptors of histamine include the H1 receptors, which are responsible for the dilation of blood vessels and the contraction of smooth muscle; the H2 receptors, which are responsible for the stimulation of heart rate and gastric secretion; and H3 receptors, which are believed to play a role in regulation of the release of histamine and other neurotransmitters from neurons. H1 and H2 receptors also mediate the contraction of vascular smooth muscle.
